1. Field of the Invention
The present invention relates to inverters. More specifically, the present invention relates to inverters of a phase shift full-bridge type.
2. Description of the Related Art
Converters are power devices for converting DC and AC voltages. AC/DC converters convert an AC signal into a DC signal, DC/DC converters convert a DC signal into a DC signal, and inverters invert a DC signal into an AC signal. Inverters are used, for example, in switching mode power supplies (SMPS).
FIG. 1 illustrates a general full-bridge inverter. The full-bridge inverter includes a power source providing input voltage Vin and four switches SA, SB, SC, and SD, diagonally coupled in pairs, for defining an AC conductive path. Further, the inverter includes a switch driver 100 containing four drivers for driving the respective switches, a PWM (pulse width modulation) unit 200 for generating pulse waveforms for driving the switch driver 100, a feedback unit 300, a protector 400, and a transformer TX. The inverter is coupled into a load CCFL.
Switches SA, SB, SC, and SD are turned on and off according to switching waveforms applied by PWM unit 200 and switch driver 100. The generated AC voltage is supplied to load CCFL through transformer TX.
PWM unit 200 comprises an oscillator, an error amplifier (Error Amp), a comparator (Comp1), and a control logic unit. The error amplifier receives a feedback signal from the feedback unit 300, and compares it with a reference voltage Vref, and the comparator compares an output of the error amplifier with a signal of the oscillator, and outputs result signals to the control logic unit. The control logic unit outputs for driving respective drivers of the switch driver 100, and comprises a toggle flip-flop (FF), a PWM latch, an XNOR gate, four OR gates, and delay circuits coupled to the OR gates.
In order to drive the inverter, switches SA, SB, SC, and SD are in some circuits turned on/off according to the phase shift full-bridge method.
FIG. 2 shows waveforms for driving the inverter by the phase shift full-bridge method. Switches SA and SB are alternately turned on/off by signals OUT A and OUT B, which are generated according to waveforms of an oscillator. Switches SC and SD are turned on/off by signals OUT C and OUT D, which are phase shifted relative to signals OUT A and OUT B and are controlled by a reference signal EA_OUT. The phase shift can be a half period. When the switches SA and SB and the switches SC and SD are concurrently turned on, the circuit becomes shorted. To prevent this, a dead time is introduced between the switching of OUT A and OUT B, and of OUT C and OUT D, so that the signals are not in the high state at the same time.
The voltage at the primary of transformer TX is transmitted to the secondary only when the switches SA/SD and SB/SC are concurrently turned on, controlled by the signals OUT A, OUT B, OUT C, and OUT D.
FIGS. 3A and 3B illustrate voltage-waveforms at point A of the left leg and point B of the right leg, and the resonant current of the LC resonance circuit C1 and L1, when the inverter is driven according to the conventional phase-shift full bridge method.
FIG. 3A shows a case of a large duty and FIG. 3B shows a case of a small duty. The term “duty” is used to describe the fraction of the cycle, when the load-voltage is on a high level. “Large duty” refers to waveforms, where the load-voltage is high for a large fraction of the cycle, “small duty” refers to waveforms with a small fraction.
For large duty the switches SA and SB in the left leg undergo zero voltage switching and the switches SC and SD in the right leg undergo zero voltage switching. But the switches SC and SD switch at higher currents than the switches SA and SB in the left leg, as shown in FIG. 3A.
When the duty is less, switches SA and SB in the left leg do not undergo zero voltage switching, but the switches SC and SD in the right leg undergo zero voltage switching. But the switches SC and SD switch at higher currents than the switches SA and SB in left leg, as shown in FIG. 3B.
Zero voltage switching provides an energy current path flowing to the secondary, when a switch is turned on after a diode coupled to a transistor is turned on. Zero voltage switching enhances the operational efficiency of MOSFETs.
Zero voltage switching reduces the operating temperature of MOSFETs. The temperature is substantially increased when no zero-current switching is performed since a large current flows to the MOSFET in the case of a low input voltage of Vin, and the temperature is substantially increased when no zero-voltage switching is performed since a high voltage is applied to the MOSFET in the case of a high input voltage of Vin.
In conventional inverters, for small duties, zero voltage switching isn't performed on the switches of the left leg and zero voltage switching is performed on the switches of the right leg. But the switches SC and SD switch at a higher current than the switches SA and SB in the left leg, as shown in FIG. 3B. Therefore, for low input voltages the switching efficiency of the right leg is low and thus the temperature of the MOSFETs of switches SC and SD becomes greater than the temperature of the MOSFETs of switches SA and SB. For high input voltages the switching efficiency of the left leg is low and the temperature of the MOSFETs of switches SA and SB becomes greater than the temperature of the MOSFETs of switches SC and SD.